Asp.Net UpdatePanel not working properly on mobile browsers

I have a checkbox and a dropdownlist (Disabled at first) in an UpdatePanel. If the checkbox is checked, then the dropdownlist become enabled.

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel" runat="server">
    <ContentTemplate>
        <asp:CheckBox ID="CheckBox" OnCheckedChanged="CheckBox_CheckedChanged" runat="server" />
        <asp:DropDownList ID="DropDownList" Enabled="false" runat="server"></asp:DropDownList>
    </ContentTemplate>
</asp:UpdatePanel>

C# Code Behind:

    protected void CheckBox_CheckedChanged(object sender, EventArgs e)
    {
        if (DropDownList.Enabled)
            DropDownList.Enabled = false;
        else
            DropDownList.Enabled = true;
    }

On Desktop browsers (Such as Chrome) it works fine. On the other hand, on a mobile browser (in this case Safari on IOS) when the checkbox is checked, nothing happens. I believe it is related to the UpdatePanel, but I'm not sure why on desktop browser it work's fine, and on Safari in IOS it won't work properly.